1. Field of the Invention
The present invention relates generally to a dispersion stabilizer for use in suspension polymerization of a vinyl compound and a method of producing the same.
2. Related Background Art
Industrially, vinyl polymers such as vinyl chloride resin are produced by suspension polymerization. In this suspension polymerization, a vinyl compound such as vinyl chloride is dispersed in an aqueous medium containing a dispersion stabilizer, and polymerization is conducted using an oil-soluble catalyst. Generally, the factors governing the quality of vinyl polymers include, for example, the degrees of conversion, the water/monomer ratio, the polymerization temperature, the type and amount of catalyst, the type of polymerization vessel, the stirring rate, and the type of dispersion stabilizer. Among these factors, the type of dispersion stabilizer has a considerable effect.
The following properties are important for a dispersion stabilizer used for suspension polymerization of a vinyl compound. (1) Addition of a small amount functions to allow vinyl polymer particles produced therewith to have a narrow particle size distribution. (2) It functions to make vinyl polymer particles produced therewith as uniform as possible and making them porous. This property is required for obtaining, for example, a vinyl polymer that can be processed easily and has a high rate of plasticizer absorption, a vinyl polymer that facilitates the removal of monomers such as vinyl chloride remaining in polymer particles, and a vinyl polymer that can prevent fish eyes from being formed in molded articles. (3) It functions to form polymer particles having a high bulk density. (4) It functions to suppress the generation of wet foam and dry foam and thereby prevent any decrease in productivity. (5) It has no harmful effect on the hue of a vinyl polymer obtained therewith. (6) It does not make an aqueous solution cloudy during charging for polymerization.
Conventionally, as a dispersion stabilizer for suspension polymerization of a vinyl compound, for instance, cellulose derivatives such as methylcellulose and carboxymethylcellulose, and partially saponified polyvinyl alcohol have been used either alone or in appropriate combinations. Conventional dispersion stabilizers, however, do not always satisfactorily meet the performance criteria described in (1) to (6) above.
For instance, polyvinyl alcohol having a polymerization degree of 2000 and a saponification degree of 80 mol % and polyvinyl alcohol having a polymerization degree of 700 to 800 and a saponification degree of 70 mol % are disclosed as dispersion stabilizers for suspension polymerization of vinyl chloride (Poval, Koubunshi Kankoukai, pp. 413–414, issued in 1984). These dispersion stabilizers, however, do not meet satisfactorily the performance criteria described in (1) to (3) above.
Furthermore, JP5(1993)-88251B describes a dispersion stabilizer that is used for suspension polymerization of a vinyl compound and is made of polyvinyl alcohol. This polyvinyl alcohol has a mean degree of polymerization of at least 500 and a ratio Pw/Pn, between a weight average degree of polymerization Pw and a number average degree of polymerization Pn, of 3.0 or lower. This polyvinyl alcohol includes a carbonyl group and a vinylene group adjacent thereto. Furthermore, a 0.1-% aqueous solution of this polyvinyl alcohol has absorbances of at least 0.3 and at least 0.15 at wavelengths of 280 nm and 320 nm in the ultraviolet absorption spectrum, respectively, and a ratio (b)/(a) of the absorbance (b) at a wavelength of 320 nm to the absorbance (a) at a wavelength of 280 nm of at least 0.30.
JP5(1993)-105702A describes a dispersion stabilizer that is used for suspension polymerization of vinyl chloride and is made of polyvinyl alcohol. This polyvinyl alcohol has a saponification degree of 75 to 85 mol % and contains 0.01 to 0.15 mol % carboxyl groups. Its 0.1-wt. % aqueous solution has an absorbance of at least 0.1 at a wavelength of 280 nm and a clouding point of at least 50° C. The above-mentioned two stabilizers made of polyvinyl alcohol, however, also do not always meet the performance criteria described in (1) to (5).
Moreover, JP8(1996)-208724 discloses a dispersant used for suspension polymerization of monomers having an ethylenically unsaturated double bond. This dispersant is made of a vinyl alcohol polymer. Its 1-wt. % aqueous solution has an absorbance of at least 2.5 at a wavelength of 280 nm. The vinyl alcohol polymer has a mean degree of polymerization of at least 500, a saponification degree of 60 to 90 mol %, a ratio Mw/Mn of weight-average molecular weight Mw to number-average molecular weight Mn of 2.5 or lower, a block character with respect to the saponification degree of 0.45 or lower, and a methanol-soluble portion of 10 wt. % or less. This dispersant exhibits properties that are balanced relatively well with respect to the requirements described in (1) to (3). However, in some cases, it does not perform satisfactorily with respect to the requirement described in (6).